Problem: What is the greatest common factor of $20$ and $45$ ? Another way to say this is: $\operatorname{gcf}(20, 45) = {?}$
Answer: The greatest common factor is the largest number that is a factor of both $20$ and $45$ The factors of $20$ are $1$ $2$ $4$ $5$ $10$ , and $20$ The factors of $45$ are $1$ $3$ $5$ $9$ $15$ , and $45$ Thus, the greatest common factor of $20$ and $45$ is $5$. $\operatorname{gcf}(20, 45) = 5$